As the metal oxide semiconductor (which will be referred to as MOS, hereinafter) is made smaller, the short channel effect becomes stronger. One of the measures for weakening the short channel effect of the transistor, is to make a MOS transistor having a surface channel structure for the purpose of enhancing its controllability by gate voltage.
In order to provide both an N- and P-channel MOS transistors in a CMOS type semiconductor device with the surface channel structure, it is necessary to make the gate electrode of the N-channel MOS transistor in the form of an N type polycrystalline silicon (Si) film and to make the gate electrode of the P-channel MOS transistor in the form of a P type polycrystalline silicon film to thereby make the work functions of the gate electrodes of both MOS field effect transistor equal to each other.
However, in the case where it is desired to have a gate electrode 13 for an N-channel MOS transistor 11 and a P-channel MOS transistor 12 as shown in FIG. 4, if a part of the gate electrode 13 corresponding to the N-channel MOS transistor 11 is made of an N type polycrystalline silicon film, while another part of the gate electrode 13 corresponding to the P-channel MOS transistor 12 is made of a P-type polycrystalline silicon film as mentioned above, N- and P-type impurities are mutually diffused within the polycrystalline silicon films.
For this reason, when the distance between the N- and P-channel MOS transistors 11 and 12 is short, the mutual diffusion causes the threshold voltages of the MOS transistors 11 and 12 to be reduced.